Recently, solid-state lighting, especially that with GaN-based wide band-gap semiconductor material, has been rapidly developed. GaN-based light-emitting devices have been widely used in the fields including display, indication, backlight source and illumination.
In GaN-based light-emitting diodes (LEDs), the epitaxial structure normally includes a p-type epitaxial layer on the top (p-side up), and can be classified as a conventional structure or a flip-chip structure depending on its light-extracting surface. The light-extracting surface of a GaN-based LED with a conventional structure is a p-type layer, and normally includes a transparent conductive layer formed on the surface of the p-type layer for enhancing current and light extraction; the light-extracting surface of a GaN-based LED with a flip-chip structure is an n-type layer, and normally includes a high-reflectivity metal layer formed on the surface of the p-type layer for enhancing current spreading and reflection. Silver (Ag), as the metal having the highest reflectivity in nature, is heavily used in flip-chip LEDs. However, Ag may cause electromigration, and since the GaN-based material is usually grown on a substrate with lattice mismatch and thermal expansion coefficient mismatch, which has a high density of dislocation defects in its crystal, the electromigration of Ag may extend to the active layer through the dislocation defects, resulting current leakage or short circuit in the LED.